Reflector for electromagnetic radiation



June 1947. c. E. McCLELLAN REFLECTOR FOR ELECTROMAGNETIC RADIATION Filed Aug. 26. 1942" INVENTOR 63/11! E. MCIe/lan.

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Patented June 17, 194'? REFLECTOR FOR ELECTROMAGNETIC RADIATION Cyril E. McClellan, Catonsville, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application August 26, 1942, Serial No. 456,211

My invention relates to reflectors for electromagnetic radiation, and in particular relates to a method of tilting or wobbling the beam of radiation from a paraboloidal reflector for ultra high frequency radio waves. For certain purposes, it is desirable to be able to send out a near cylindrical beam of radiant energy, preferably of the ultra high frequency radio wave type, and to be able to vary the direction of this beam periodically, oftentimes at a rather high rate. An example of this is in systems in which a beam of short wave radiant energy is sent out and moved about through a limited angle, so that, if it is reflected from any metallic body, such as an approaching aircraft or a ship, the reflected Waves can be picked up by a conveniently located receiving device near the transmitter and the presence of the reflecting body thus discovered. Hitherto various methods have been used of varying or wobbling the position of the beam in order to scan a substantial area. Such movement of the beam i often called lobe-switching. One such prior art method has consisted in rotating the original source of the radiation in a circle or other closed path surrounding the focus of a paraboloidal reflector which is used to produce a concentrated beam. When the radiating source has linear dimensions which are not too great compared with the focal length of the paraboloidal reflector, and is located exactly at the focal point of the latter, a beam which is substantially cylindrical about the axis of the paraboloid will be sent out. If now the radiating source is moved to one side of the focal point, it can be shown that the beam, while still approximating a cylindrical form, has its axis displaced in the opposite direction from the axis of the paraboloid. One of the prior art methods of tilting or wobbling the beam thus consisted in periodically rotating the radiation source in some closed path in the focal plane about the focus of the reflector. In most practical cases, it is necessary to employ a concentric transmission line to carry ultra high frequency current to the radiating source which usually consisted of a small dipole radiator, and this occasioned serious difficulties because of the necessity for providing a high-speed rotating joint in the concentric transmission line.

Another prior art method which avoided the difficulty just mentioned consisted in keeping the dipole stationary, but rotating the paraboloidal reflector about the dipole which was itself displaced from the axis of the paraboloid. This method also involved considerable practical difii- 4 Claims. (Cl. 250-11) culties in providing a rotating carriage for the reflector.

I have discovered that it is possible to produce a similar wobbling or tilting of the beam from the reflector while maintaining both the reflector itself and the dipole stationary relative to each other by providing a suitably formed dielectric body which is positioned in the path of the radiant energy and is periodically moved in the necessary fashion to periodically tilt the beam in the desired path.

One object of my invention is accordingly to provide a reflector for radiant energy with means for varying the direction of the beam emanating therefrom without moving the source of radiant energy relative to the reflector which cooperates with it.

Another object of my invention is to provide a paraboloidal reflector having a radiator of ultra high frequency electric waves as an emanating source of radiant energy for a beam to be projected therefrom, with means for varying the direction of the beam relative to the axis of the paraboloidal reflector while maintaining the relative position of the emanating source and the reflector fixed.

Still another object of my invention is to provide a reflector of radiant energy with a movable dielectric interposed in the path of at least part of the energy emanating therefrom.

Still another object of my invention is to provide a paraboloidal reflector having an electromagnetic dipole which acts as an energy source producing a concentrated beam therefrom with a movable dielectric which intercepts the paths of radiant energy therein, and when rotated rotates the beam emanating from the reflector in a circular path about the reflector axis.

Other objects of my invention will become apparent upon reading the following specification taken in connection with the drawing, in which:

Figure 1 is a diagram used in explaining the principles of my invention.

Fig. 2 shows a View, partly in elevation and partly in cross-section, of one reflector system made in accordance with one species of my invention; and

Fig. 3 shows a similar view of a reflector system embodying another species of my invention.

Referring to Fig. 1, a reflecting surface I having the form of a paraboloid of revolution about the central axis 2 has a focal point 3. If radiant energy is emitted from the point 3, it will be reflected from the surface I in the form of a substantially cylindrical beam having the crosssection of the aperture of the surface I. It can be shown that the aperture plane of the paraboloid is traversed by radiation of uniform phase at any instant. If now the radiator is deflected directly upward by the distance d, a change in phase of the radiation will occur at various points over the area of the aperture plane, the amount of this phase change varying in a uniform manner from point to point. The amount of phase change at any particular point can be calculated by determining the change in the length of the path which the radiation traverses in going from the radiator to the reflector and thence to a particular point on the aperture plane. Thus, consider a quantum of radiation starting from the focus 3 in a path having an angle A relative to the axis 2 and an angle B with the vertical axis 4. This radiation will pass through the point on the aperture plane and will constitute the entire radiant energy at that point. It can be shown mathematically that the length of the path traversed by the radiation from the point 3 to the point 5 in the aperture plane is independent of the values of A and B, and hence that, as stated above, the phase of the radiation is uniform all over the aperture plane. If now the source be displaced as above-mentioned from the focus 3 by a distance d in the focal plane, it can be readily shown that the length of the path of radiation to the reflector and thence to the aperture plane will change from that just calculated, and that the amount of this change will vary with the values of A and B. Calculation shows that where the distance d is small compared with the focal length of the reflector, the change in the length of the path is approximately d sinA cosB from which it follows that the change in phase of the radiation arriving at the aperture plane plane will be 211 (qr/k) sin A cos B, where A is the length of the radiation. This quantity will obviously vary with the values of A and B. It may be pointed out that displacement of the point radiation source from the focal point will tilt the axis of the beam away from the axis of the reflector by an angle of the order of tanwhere j is the focal length of the reflector; that the angle through which it is desired to thus move the axis of the beam from the reflector axis is usually quite small in practice, and hence the assumption made in the foregoing calculation that d is small relative to f is justifiable.

Since movement of the source transversely to the reflector axis is known to tilt the reflected beam, and has been shown to produce a phase change of the radiant energy at the aperture plane, it is legitimate to consider the tilting of the beam to be the result of the above calculated change of phase of radiant energy of the aperture plane; and from this it follows that any other way in which the same change of phase of radiant energy over the area of the aperture plane is produced will result in the same tilting of the reflector beam.

When a wave of radiant energy passes through a dielectric of thickness t and dielectric constant n, it will undergo a change of phase of the amount 2(1r/A)t(n1) from its phase before the dielectric was instituted for an air path. It accordingly follows that it will be possible by interposing dielectric of proper dielectric constant and thickness in the path of all the radiation which crosses from the reflector aperture plane to produce the same tilt in the beam as would otherwise result if the radiation source were displaced by a distance din the focal plane from the focal point of the reflector. Figs. 2 and 3 show different arrangements for interposing dielectric of proper thickness in the path of the radiant energy.

Referring specifically to Fig. 2, a reflector I having a dipole 2' located at its focal point 3 is provided with a slab 6 of dielectric material such for example as,

1. Polystyrene 2. Glass 3. Wax

4. Wood and etc.

which covers the circular area of its aperture and which varies in thickness from point to point of that area in the manner which will now be calculated. If we consider any point located similar to 5 in Fig. l the radiant energy traversing it may be considered as emanating from the focus 3 at an angle A relative to the reflector axis and at an angle B relative to the vertical plane t. It has just been shown that there is some thickness t of the dielectric B at the point 5 which will produce the same phase change in the radiant energy passing out of the reflector as will a displacement of the light source vertically upward from focal point 3 by a distance d. This thickness is determined by equating the expression given above for the phase change due to the displacement d, and the expression given above for the phase change due to the presence of a dielectric of thickness t; that is to say 21r(n1)t 21rd sin A cos B which gives for the value of dielectric thickness at any point 5,

15:11 sin A cosB n-l By finding the values of A and B which correspond to the location of any point 5 on the aperture plane, which can be done by mathematical methods well known, it is thus possible to calculate the thickness of the dielectric 6 for every point of its area.

It will be noted that it is possible to employ a dielectric of constant thickness t but varying index of refraction n from point to point; or one in which both n and t vary so long as the above equation is fulfilled. If one of these quantities is given arbitrary values the equation may be solved to obtain an expression for the other.

The value of d in the foregoing equation is readily determined if the tilt of the beam relative to th central axis of the reflector is known, since d=f tan C=fC approximately, where C is the desired angle of tilt between the reflected beam and the axis of the reflector.

It will be observed that since B will in general vary through all values between 0 and 360, cos B will sometimes be negative, and that consequently the value of the dielectric thickness t will correspondingly have negative values. This difficulty may be avoided practically by adding a uniform amount to the thickness of the dielectric all over its surface, this amount being such that t will never have a negative value. Such a procedure will merely change the phase of the radiation by a uniform amount over the cross-sectional area of the beam and will not in any way affect the tilt of the beam. correspondingly, the correct expression for the thickness of the dielectric 6 to be used in practice is,

t= (l +sin A cos B) By rotating the dielectric member 6 about the axis of the reflector, the beam may be caused to rotate similarly about the axis of the reflector, and the end of the beam will thus describe a circular path. The dielectric 6 may be rotated by any convenient arrangement such, for example, as the provision of encircling holder 1, provided at intervals about its periphery with small pivots 8 which may carry rollers 9, the latter in turn bearing upon a circular track ll formed near the rim of the reflector l. The holder 1 may have gear teeth out in its outer edge and be turned by a small gear Wheel l2 mounted in a bearing [3 which is attached to the reflector I.

The foregoing arrangement will result in the rotation of the end of the beam in a circular path. It is obvious that by providing more complex arrangements the center of the dielectric 6 may be made to move in a closed path of any other form, and this will result in the end of the beam following a closed path of similar form.

In the arrangement of Fig. 2, the dielectric 6 was interposed in the path of the radiation just after it traversed the aperture plane of the reflector I. However, it is immaterial at what point in the path of the radiation the dielectric is interposed, and Fig. 3 shows an arrangement in which the dielectric is interposed in the radiation path before the radiation strikes the reflector I. In Fig. 3, the radiating dipole 2' is positioned at the focal point 3 of the reflector l and is supplied by a concentric line M of a form well known in the art which is fixed in position along the extension of the axis of the reflector. Surrounding the line H is a hollow sleeve l5 which supports at its inner end a dielectric body [6. The thickness of the dielectric body IS in the path of any quantum of radiation emitted by the radiant source at 3 is made to vary in accordance with the formula last given above, the values of A and B being obviously determinable in the same way as has been mentioned in connection with Fig. 2. The sleeve I5 passes through a hole at the butt of the reflector l and is supported for rotation in a hearing I! which is attached to the reflector I. The sleeve 15 may be provided with any convenient arrangement for rotating it in the bearing IT; for instance, it may be provided with a pinion [8 which meshes with a gear l9 supported in a bearing 2| affixed to the reflector l.

While I have described my invention as used in connection with a paraboloidal reflector for producing a substantially cylindrical beam, it will be recognized by those skilled in the art that its principles are of broader application and that it is within the purview of my invention to position and, if desired, move dielectric bodies in the path of radiation between radiant sources generally and reflectors of other geometrical form than paraboloids; for example, ellipsoids, hyperboloids and spheres.

I claim as my invention:

1. In combination with a reflector having the form of a paraboloid of revolution, a source of radiant energy positioned substantially at the focal point of said reflector, and an element of dielectric material positioned to intercept the path of all radiation emitted by said source and incident upon said reflector, the thickness of said dielectric along the path traversed by radiation having the value where n is the dielectric constant of the dielectric, K is a constant, A is the angle made by the radiant path with the reflector axis, and B is the angle made by the radiant path relative to a plane passing through the axis of the reflector.

2. In combination with a reflector of such form as to have a focal point, a source of radiation located at said focal point and a dielectric positioned to intercept the paths of radiation emitted by said source and incident upon said reflector, the thickness of the dielectric along said paths having the value where n is the dielectric constant of the dielectric, K is a constant, A is the angle made by the radiant path with the reflector axis, and B is the angle made :by the radiant path relative to a plane passing through the axis of the reflector.

3. In combination with a reflector having the form of a paraboloid of revolution, a source of radiant energy positioned substantially at the 0. cal point of said reflector, and an element of dielectric material positioned to intercept the path of all radiation emitted by said source and incident upon said reflector, the thickness of said dielectric along the path traversed by radiation having the value (1+sin A cos B) where n is the dielectric constant of the dielectric, K is a constant, A is the angle made by the radiant path with the reflector axis, and B is the angle made by the radiant path relative to a plane passing through the axi of the reflector, and. means for revolving said dielectric about the axis of the paraboloid.

4. In combination with a reflector of such form as to have a focal point, a source of radiation located at said focal point and a dielectric positioned to intercept the paths of radiation emitted by said source and incident upon said refiector, the thickness of the dielectric along said paths having the value where n is the dielectric constant of the dielectric, K is a constant, A is the angle made by the radiant path with the reflector axis, and B is the angle made by the radiant path relative to a plane passing through the axis of the reflector, and means for revolving said dielectric about the axis of the paraboloid.

CYRIL E. MCCLELLAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,083,242 Runge June 8, 1937 FOREIGN PATENTS Number Country Date 448,254 Germany Aug. 8, 1927 

